The present invention, relates to a method and device for fitting pull-off tabs to pourable food product packaging material.
Many pourable food products, such as fruit juice, UHT (ultra-high-temperature treated) milk, wine, tomato sauce, etc., are sold in packages made of sterilized packaging material.
A typical example of such a package is the parallelepipedal package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is formed by folding and sealing laminated strip packaging material. The packaging material has a multilayer structure comprising a layer of fibrous material, e.g. paper, covered on both sides with layers of heat-seal plastic material, e.g. polyethylene. In the case of aseptic packages for long-storage products such as UHT milk, the packaging material comprises a layer of barrier material, e.g. an aluminium film, which is superimposed on a layer of heat-seal plastic material and in turn covered with another layer of heat-seal plastic material defining the inner face of the package eventually contacting the food product.
As is known, packages of the above type are produced on fully automatic packaging machines, on which a continuous tube is formed from the packaging material supplied in strip form. The strip of packaging material is sterilized on the packaging machine, e.g. by applying a chemical sterilizing agent, such as a hydrogen peroxide solution; following sterilization, the sterilizing agent is removed, e.g. vaporized by heating, from the surfaces of the packaging material; and the strip of packaging material so sterilized is kept in a closed sterile environment, and is folded and sealed longitudinally to form a vertical tube.
The tube is filled with the sterilized or sterile-processed food product, and is sealed at equally spaced cross sections at which it is then cut into pillow packs, which are subsequently folded mechanically into finished, e.g. substantially parallelepipedal packages.
The finished package has a pull-off tab which is fitted to the packaging material before the material is sterilized and folded and sealed to form the vertical tube.
More specifically, a through opening is first formed in the packaging material at a cutting station. A so-called xe2x80x9cpatchxe2x80x9d, defined by a small sheet of heat-seal plastic material, is then heat-sealed over the opening, on the side of the packaging material eventually defining the inside of the packagexe2x80x94this is done at two successive sealing stations to prevent overheating. And at this point, the pull-off tab is fitted to the opposite side of the packaging material and heat-sealed to the patch at a further sealing station.
The tab comprises a layer of aluminium and a layer of heat-seal plastic material, normally polyethylene, which is sealed to the patch. By virtue of the tab and patch adhering to each other, the portion of the patch sealed to the tab is removed together with the tab, thus uncovering the opening, when the tab is pulled off.
The patch and tab are normally heat-sealed to each other using a so-called xe2x80x9chot-plate sealingxe2x80x9d process, whereby the patch and tab are blown by compressed air against a heated plate.
Alternatively, as described for example in Patent EP-B-149130 filed by the present Applicant, a heat-seal process commonly known as xe2x80x9cinduction sealingxe2x80x9d is also used, whereby the packaging material at the opening is pressed between a heating element on the patch side and a backing plate on the tab side. The heating element comprises a central compressed-fluid, e.g. compressed-air, supply conduit to push the patch onto the tab and against the backing plate; and an inductor inserted in the work surface of the heating element to induce an electric current in the material held by the compressed fluid against the backing plate.
In short, regardless of the sealing technique used, the above method therefore consists in removing portions of the packaging material to form the openings, and then xe2x80x9crepairingxe2x80x9d the packaging material by applying the patches, thus resulting in poor efficiency and relatively high cost.
Moreover, though ensuring troublefree opening of the packages, the above method poses several problems as regards the integrity of the packages, and in particular as a result of the superheated patch interacting with the normally jagged edges of the opening formed in the layer of fibrous material.
One possible alternative is to form the through opening directly in the layer of fibrous material of the packaging material before the fibrous layer is combined with the plastic and aluminium layers, hereinafter referred to simply as xe2x80x9clamination layers.xe2x80x9d
At the end of the lamination process, therefore, the opening is covered by the lamination layers; the package is perfectly integral; and no patch is required.
As in the previous case, the tab is fitted to the side of the packaging material eventually defining the outside of the package, and is sealed to the layer of plastic material covering the opening.
To ensure troublefree opening of the packages, the tab and the lamination layers sealed to it must adhere firmly. In the case in question, however, the degree of adhesion provided for by conventional heat-seal methods as described above is fairly poor, so that, when the tab is pulled off, parts of the lamination layers remain inside the opening, thus preventing smooth outflow of the food product through the opening.
It is an object of the present invention to provide a method of fitting pull-off tabs to pourable food product packaging material, designed to eliminate the aforementioned drawbacks typically associated with known methods.
According to the present invention, there is provided a method of fitting pull-off tabs, made of heat-seal material, to strip packaging material having a number of through openings and covered, on the opposite side to that to which said tabs are fitted, with at least a first and a second lamination layer made respectively of heat-seal material and electrically conducting material and covering said openings; the method being characterized by comprising the step of joining each said tab to said packaging material at at least one respective said opening by simultaneously performing hot-plate sealing and induction sealing operations.
According to the present invention, there is also provided a device for fitting pull-off tabs, made of heat-seal material, to strip packaging material having a number of through openings and covered, on the opposite side to that to which said tabs are fitted, with at least a first and a second lamination layer made respectively of heat-seal material and electrically conducting material and covering said openings; the device being characterized by comprising hot-plate sealing means and induction sealing means, which are activated simultaneously to join each said tab to said packaging material at at least one respective said opening.